Plasma decomposition of CO2 in the presence of metal catalysts

Abstract

The decomposition of CO2 in fan-type ac glow discharge plasma reactors coated with gold, copper, platinum, palladium, rhodium, and mixed rotor/stator systems (Au/Rh and Rh/Au) was investigated. A high-voltage ac signal was used to produce a plasma between the fins of a turning rotor and an immobile stator, through which a 2.5% CO2 in He mixture was passed. The analysis of the product gases was achieved using a mass spectrometer equipped with a partial pressure analyzer, and the decomposition of CO2 was found to proceed to CO and O2 with >80% selectivity. The percentage conversion of CO2 increases with decreasing flow rate and increasing input voltage. The opposite trend is obtained when the energy efficiency is evaluated. Spectroscopic data indicate that the diluent gas plays a role in the dissociation of CO2, likely via charge and energy transfer from excited state He species to produce vibrationally excited CO2+ intermediates. The order of reactivity for the different metal catalyst coatings is Rh > Pt ≈ Cu > Pd > Au/Rh ≈ Rh/Au ≈ Au. With the Rh-coated reactor, conversions as high as 30.5%, reaction rates of 8.07 × 10-4 mol/h, and energy efficiencies of 3.55% could be obtained. There is a clear relationship between excitation temperature, Tex, of a pure He plasma and the conversion of CO2 in a CO2/He plasma: decreasing Tex corresponds to increasing conversion. © 1998 Academic Press.